The present invention relates generally to magnet assemblies suitable for use in impact printers of the type utilizing moving coil hammers. Several United States patents have issued disclosing this type of impact printer. For example only, attention is called to: U.S. Pat. Nos. 3,087,421, 3,172,352, 3,279,362, 3,285,166, and 3,643,595.
In all of the printers disclosed in the foregoing patents, a permanent magnet assembly is provided which defines a plurality of gaps, each of which receives a flat coil which is physically coupled to a hammer for impacting against a type bearing surface. In at least some of the foregoing patents, the magnet assembly is comprised of a plurality of permanent magnet members arranged in first and second parallel rows and poled so as to create a closed magnetic field path in which the flux is oriented in one direction in the first row and in an opposite direction in the second row. Gaps in the first and second rows are aligned such that each pair of aligned gaps receives a different hammer coil. A current driven through the coil produces a force on the coil which propels the hammer toward the type bearing surface, which may comprise a moving drum or band.
The force developed on the coil is proportional to the product of the flux density (B) within the gap and the current through the coil (i). As a practical matter, the magnitude of the coil current (i) should be minimized to avoid heating problems. Thus, for a given value of coil current, the force developed on the hammer will be related directly to the magnitude of flux density within the gap. For a given geometry of certain dimensions, the flux density is dependent primarily upon the permanent magnet material selected. Generally speaking, higher energy materials, such as rare earth materials, are considerably more costly than lower energy materials such as Alnico.